The two main categories of atomic clocks include 1) radio frequency (RF) or microwave clocks and 2) optical clocks. The RF clocks have many forms and include passing atoms through microwave fields to drive the atom into a different atomic state, detecting the state change, and then using that information to control or discipline a microwave source (usually a quartz oscillator). The three most common commercial microwave clocks are rubidium vapor cell clocks, cesium beam tubes, and hydrogen masers.
Optical clocks also come in many different forms spanning the use of vapor cells, thermal beams, and cooled ensembles of atoms. The fundamental principle of an optical atomic clock is that a laser is frequency locked to a specific atomic transition and then the laser is used to discipline a frequency comb. The frequency comb is a pulsed laser whose repetition rate can be used for timing purposes when disciplined by the frequency standard that is referencing the atoms.
Optical clocks boast higher performance than RF clocks but have not been developed for use outside of a laboratory for several reasons. The primary reason is that the laser systems usually require laboratory-like environments and large amounts of power. Also, an optical clock that requires laser cooled atoms typically requires multiple laser systems and a complex and sensitive optical arrangement. While thermal beam optical clocks can benefit from using a single laser rather than multiple laser systems, thermal beam optical clocks typically require regular servicing and have a limited supply of atoms. Also, for the highest performance, a thermal beam optical clock may require optical alignments that are not resilient enough to operate outside of a laboratory environment.
Vapor cell optical atomic clocks may provide a desired trade-off between performance and environmental sensitivity because they rely on a single laser, the atoms remain in the system indefinitely, and they do not require particularly stringent optical alignments. However, conventional optical atomic clock designs require either iodine or rubidium, neither of which may meet performance objectives because of the properties of the atomic or molecular transitions. Approaches for containing atoms that offer higher performance in a vapor cell appear to be lacking.
Alkaline-earth atoms are known to possess spectrally narrow electronic transitions that can be accessed with visible laser sources. Such alkaline-earth atoms include calcium, strontium, magnesium, barium, radium, zinc, cadmium, and ytterbium, for example. These narrow transitions form the basis for the world's most precise clocks and have other appealing metrological features that find application in magnetometry, atom interferometry, formation and study of Bose-Einstein Condensates, and searches for physics beyond the Standard Model of elementary particles.
However, accessing these transitions requires a dense and sometimes cold source of atomic vapor, which may be challenging due to the low vapor pressure of these atoms. Scientific literature shows that certain glasses and crystalline materials may be able to contain vapors of these atoms at the requisite temperature without damage. However, forming a pure environment free from background particles may be difficult due to the inability to have a fill port through which the background gas could be removed, and the desired alkaline-earth material could be added.
A second concern for a potential vapor cell is that many of the above applications require multiple degrees of optical access, including (in the case of a vapor cell whose shape is a rectangular prism) from all six faces. Multiple degrees of optical access can be important for providing access to a clock laser, for observing performance attributes, and for certain applications like magneto optical traps, which require optical access from six sides of the cell. Providing multiple degrees of optical access is generally incompatible with the need to heat and insulate the vapor cell to support the needed temperature (typically 400-600 degrees Celsius) to form a vapor.
Therefore, a need exists for a technique by which a pure atomic vapor source may be developed inside of a compact cell, capable of supporting residual background pressures on the order of 10−6 to 10−8 Torr, for example, so that the electronic transitions associated with alkaline earth atoms can be accessed for such applications like optical atomic clocks. Also, a need exists for a technique by which a compact vapor cell may be heated to the desired temperatures without severely diminishing optical access to the vapor.
This background information is provided to reveal information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding description constitutes prior art against the present invention.